The present invention generally relates to a drive shaft coupling device and, in particular, relates to a coupling device which includes two shaft segments magnetically coupled to each other.
In general, drive shafts are used to transfer torque from a drive motor, or other source of rotational force, to a load. Most frequently, drive shafts are unitary, straight and designed toward a particular application or environment. However, in many instances the axis of the shaft of the motor or driving mechanism and the rotational axis of the load are misaligned, either angularly or linearly. In order to overcome such misalignment, segmented drive shafts are useful. However, as the segments are usually flexibly coupled to overcome misalignments, a major consideration becomes the efficient transfer of the driving torque to the load.
In the general field of analytical instruments, drive shaft misalignment, if not corrected or compensated for, can result in inaccurate results and/or the misleading characterization of the sample being analyzed. The difficulties of shaft misalignment in analytical instruments generally arise in the control of the light chopper which divides a single light beam into a sample beam and a reference beam. Conventional manufacturing techniques often result in the misalignment of the drive motor and the rather massive chopper. Consequently, if a unitary shaft is employed, the rotation of the chopper becomes asymmetrical, resulting in erroneous sample characterizations.
In addition to the problems resulting from shaft misalignment, the transference of torque is also a problem. That is, in analytical instruments, the size of the drive motor is maintained as small as possible for manufacturing and commercial convenience. However, this presents a problem in that the inertial mass of conventional chopper wheels is often so great that the chosen drive motor cannot bring the chopper up to speed. Thus, when switched on, the drive motor usually needs to start the chopper rotating relatively slowly, i.e., apply a comparatively large torque, to overcome the inertia and subsequently apply less torque to maintain the chopper at its operating speed. Thus, some form of a clutch mechanism is usually necessary.
One solution to the above-stated problem is described in U.S. patent application Ser. No. 128,289 filed Mar. 7, 1980, entitled "A Spectrophotometer System Having Power Line Synchronization" and assigned to the assignee hereof. As described therein, a segmented drive shaft between a motor and a chopper is coupled by means of a pair of orthogonal pins arranged to transmit torque along the shaft. These pins are necessary, since the magnetic coupling employed therein is insufficient to overcome the inertia of the chopper by itself. The magnets are ostensibly used in that design to overcome any phasing or oscillation errors during chopper rotation. This is achieved by the attracting/opposing action of poled magnets.
Although the arrangement thus described overcomes many of the recognized difficulties, it has, in itself, numerous drawbacks. One such drawback is that the mechanical pin connection between the drive shaft segments produces a degree of bounce, as well as a considerable wear factor in the long term operation of the apparatus. Further, the magnets employed must be substantially identical in physical size and magnetic strength in order to avoid osscillations or bouncing during operation. This results in an unnecessary expense.